In the U.S., about one-quarter of the nation's electricity budget is spent on lighting, or more than $37 billion annually. Most of this expense results from the use of conventional incandescent and fluorescent lighting. Unfortunately, the efficiency of current incandescent and fluorescent lamps plateaued many years ago at 5% and 20%, respectively. Due to the large electricity cost associated with conventional incandescent and fluorescent light sources, much attention has focused recently on more energy efficient lighting alternatives. One alternative is provided by Solid State Lighting (SSL). SSL is a lighting technology that uses light emitting diodes (LEDs) for white light general illumination. SSL has the potential to offer enormous economic and environmental advantages over conventional lighting technologies for general illumination, promising efficiencies in excess of 50%. In addition, SSL offers advantages in color tunability, design flexibility, robustness, long lifetimes, mercury-free manufacturing, and fast turn on times.
While there exist white light SSL solutions for general illumination, none are suitable for widespread use today. In fact, current roadmaps project SSL will not supplant current lighting alternatives in earnest until 2020, resulting in continuing energy wastage for almost two decades. Much of this is related to the current high cost of solid state lamps—currently two orders of magnitude more expensive than conventional lighting sources. However, there are some performance issues as well. The most common SSL technologies are based on bulk inorganic LEDs (light emitting semiconductor chips). There are two main categories. The first category includes phosphor based systems where phosphors are used to convert the output from an LED light source into visible light. The most common phosphor systems use a blue LED in combination with a yellow phosphor. In these systems the blue light from the LED excites the phosphor which emits yellow light. The yellow light mixes with the blue light from the LED to produce a whitish light. In another common phosphor based system, light from an ultraviolet (UV) LED is used to excite red, green, and blue phosphors to produce a whitish light. The second category of SSL devices is based on the use of multiple LEDs. In these systems at least two LEDs (e.g. blue and yellow) or three LEDs (e.g. red, blue, and green) are used in combination. When the LEDs emit, the light from the different LEDs mixes to create a whitish light. FIG. 1 shows schematic diagrams illustrating each of the SSL systems described above.
Unfortunately, SSL illumination devices of the type described above have met with limited commercial success because they produce white light with a poor color rendering index (CRI). CRI describes a light source's ability to accurately render the colors of the objects it illuminates. CRI has a universal measurement system numbering from 0 to 100, where the culturally accepted incandescent bulb sets the standard at 100. Generally, the higher the CRI the better the color rendering properties of the light. The CRI for presently available SSL devices is around 75 or lower. Another disadvantage of the SSL white light LEDs presently available is their relatively poor efficiency. White light LEDs are currently far from their promised efficiency, and the efficiencies that have been achieved by certain monolithic devices. In most cases, SSL devices operate in the 10-20 lumens/watt (lm/w) range which is too low to make these devices an attractive alternative to conventional lighting technologies—particularly on a cost-benefit basis. In addition the SSL white light LEDs presently available suffer from lifetime problems, color shifting over time, and differential aging of the different color LEDs and phosphors used to generate the white color. SSL devices based on the multiple LED approach further suffer from the drawback that they involve complex drive electronics to compensate for the differential aging and color shifting inherent in each of the different compounds used to make up the three types of LEDs.
The phosphors used in the SSL devices described above are materials capable of absorbing energy and reemitting the energy in the form of light, having a longer wavelength than the absorbed energy. This process is known as luminescence. A variety of phosphor materials are known. Most of these are made from inorganic materials, such as zinc sulfide or yttrium oxides that have been doped with transition or rare earth metals. U.S. Pat. No. 6,501,091 describes light emitting diodes made from quantum dots of Group II-VI semiconducting materials embedded in a matrix have been proposed as phosphors in SSL illumination devices. However, that patent does not disclose the use of Group IV nanoparticles in an illumination device. Nor does the patent disclose the production of white light having a high CRI value.
Thus a need exists for an efficient and inexpensive white light LED system that is capable of producing white light with a high CRI without substantial color shifting, lifetime, or differential aging problems.